1. Field of the Invention
The present invention relates generally to piezoelectric devices, and more particularly to a piezoelectric transformer supplied with an input a.c. current.
2. Description of the Related Art
Wound-type electromagnetic transformers have been used for generating high voltage in the internal power circuits of devices such as television deflectors or chargers for copiers which require high voltage. Such transformers consist of a conductor wound onto a magnetic core. Because a large number of turns are required to realize a high transformation ratio, transformers that are compact and slim in shape are extremely difficult to produce.
To remedy this problem, piezoelectric transformers utilizing the piezoelectric effect have been developed. FIG. 1 illustrates a Rosen-type piezoelectric transformer, according to Ohnishi, U.S. Pat. No. 5,806,159, A plate of a piezoelectric material 102 has upper and lower input electrodes 104 and 106 which define a driving or input region 108 of the piezoelectric plate 102. The remainder of the plate 102 constitutes a generator or output region 110 with an output electrode 112 at its end. The input region 108 is polarized orthogonal to the electrodes 104 and 106, as indicated by arrow 114 in the figure, while the output region 110 is polarized orthogonal to electrode 112, as indicated by arrow 116.
This piezoelectric transformer operates as follows: When a voltage is impressed across input electrodes 104 and 106 from external leads 118 and 120, an electric field increases in the direction of polarization, and a longitudinal vibration in the transverse direction parallel to electrodes 104 and 106 is excited by the piezoelectric effect, displaced in a direction perpendicular to polarization, known as the piezoelectric transverse 31 effect, causing the entire transformer to vibrate. Moreover, in the output region 110, due to the piezoelectric effect generating a potential difference in the polarization direction due to a mechanical strain in the polarization direction, a voltage is produced which has the same frequency as the input voltage from output electrode 112 to external lead 122. At this time, if the voltage input frequency is made equal to the resonant frequency of the piezoelectric transformer, a high output voltage can be obtained.
This piezoelectric transformer is used in a resonant state. Compared with ordinary electromagnetic transformers it has numerous advantages, including: 1) a compact and slim shape that can be achieved because a wound-type construction is not required and energy density is high; 2) the potential for non-combustibility; and 3) a lack of electromagnetic induction noise. Furthermore, the Rosen piezoelectric transformer is monolithic, which gives it an advantage over multi-layer devices in that it does not suffer from bonding problems such as a reduction in efficiency due to softening of the bonding layer at high temperatures.
In Rosen and other types of conventional piezoelectric transformers, a rectangular waveform input, (having a harmonic content of a series of sine waves according to the Fourier transform fo+3fo+5fo . . . +nfo), produces an output having a sine waveform of only the fundamental frequency. This can be a disadvantage because the rise and fall time of a sine wave is much slower than that of a rectangular wave, and fast rise and fall times are important for driving transistor switches OFF and ON (such as in DC-DC power converters), since significant power losses occur in the transistors during the transition between the OFF and ON states.
Moreover, in Rosen and other conventional piezoelectric transformers the input and output regions are not entirely electrically separated, due to a parasitic capacitance between the input and output regions.
This invention provides a piezoelectric transformer and operating method capable of passing the fundamental and third harmonic frequencies of a rectangular wave input and with a reduced parasitic capacitance.
The new piezoelectric transformer has a monolithic planar structure in the form of a thin rectangular piezoelectric plate having a uniform polarity orthogonal to its major surfaces. Pairs of input and output electrodes are formed on the top and bottom surfaces of the piezoelectric plate. Since the device is monolithic, it does not suffer from bonding problems (e.g. bond elasticity) inherent with multi-layer devices. However, it is also possible to stack transformer layers if a higher output current is desired.
A three electrode pair embodiment enables the device to pass the fundamental and third harmonic frequencies of a rectangular wave input, producing a pseudo-rectangular wave output. A pseudo-rectangular wave has a much faster rise and fall time then a sine wave, although somewhat slower then a true rectangular wave.
A piezoelectric transformer according to the present invention is further capable of producing multiple isolated outputs of either polarity. It is well suited for driving both capacitive loads such as the input gate of a MOSFET power transistor, and other loads such as resistive or resistive/capacitive circuits.
Parasitic capacitance is minimized by grounding leakage current between the input and output regions. This is accomplished by placing a grounding element such as a grounded guard electrode between the input and output regions. In a monolithic transformer the grounding element can be a thin electrode disposed in a continuous band around the surface of the piezoelectric body, between the input and output regions. This can reduce the stray capacitance by a factor of 10 to 20, and the effective input-output coupling capacitance can be reduced to 1-5% of the input capacitance, depending upon the dielectric constant of the piezoelectric material.
Moreover, in comparison to electromagnetic transformers, the new piezoelectric transformer is compact, simple to fabricate, low cost, and immune to magnetic interference.